FIG. 1 is a structural diagram illustrating a Long Term Evolution (LTE) system which is a mobile communication system. The LTE system is an evolved version of a conventional UMTS system and has been standardized by the 3GPP (3rd Generation Partnership Project).
The LTE network may be generally classified into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN includes at least one eNode-B serving as a base station and an Access Gateway (AG) located at the end of the network so that it is connected to an external network.
The AG may be classified into a portion that handles user traffic and a portion that handles control traffic. A first AG that handles user traffic may communicate with a second AG that handles control traffic via a new interface. A single eNode-B may include at least one cell.
A first interface for transmitting user traffic or a second interface for transmitting control traffic may be located between several eNode-Bs. The CN includes the AG and a plurality of nodes for registering users of User Equipment (UEs). If required, another interface for discriminating between the E-UTRAN and the CN may also be used for the LTE network.
FIG. 2 is a conceptual diagram illustrating a control plane of a radio interface protocol structure between the UE and the UTRAN (UMTS Terrestrial Radio Access Network) based on the 3GPP radio access network standard. The radio interface protocol horizontally includes a physical layer, a data link layer and a network layer. The radio interface protocol vertically includes a User Plane for transmitting data and a Control Plane for transmitting a control signaling.
The protocol layers shown in FIG. 2 may be classified into the first layer (L1), the second layer (L2), and the third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model that is well known in the art.
The physical layer which is as the first layer (L1) provides an Information Transfer Service over a physical channel. A radio resource control (RRC) layer located at the third layer (L3) controls radio resources between the UE and the network.
The RRC layer exchanges RRC messages between the UE and the network for this purpose. The RRC layer may be distributed to a plurality of network nodes, such as eNode-B and AG and may also be located at the eNode-B or the AG.
A radio protocol control plane will be described with reference to FIG. 2. The radio protocol control plane includes a physical layer, a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer and a Radio Resource Control (RRC) layer.
The physical layer transmits an Information Transfer Service to an upper layer over a physical channel while acting as the first layer (L1). The physical layer is connected to a Medium Access Control (MAC) layer located thereabove via a transport channel.
The MAC layer communicates with the physical layer over the transport channel such that data is transferred between the MAC layer and the physical layer. Data is transferred between respectively different physical layers, such as between a first physical layer of a transmitting side and a second physical layer of a receiving side.
The MAC layer of the second layer (L2) provides services to the RLC (Radio Link Control) layer, located thereabove via a logical channel. The RLC layer of the second layer (L2) supports transmission of data with reliability It should be noted that the RLC layer is depicted in dotted lines, because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist.
The RRC (Radio Resource Control) layer located at the lowermost part of the third layer (L3) is only defined in the control plane. The RRC layer handles the control of logical channels, transport channels and physical channels in relation to configuration, reconfiguration and release of Radio Bearers (RBs). An RB refers to a service that is provided by the second layer (L2) for data transfer between the UE and the E-UTRAN.
FIG. 3 is a conceptual diagram illustrating a User Plane of a radio interface protocol structure between the UE and the UTRAN according to the 3GPP radio access network standard. The radio protocol user plane includes the physical layer, the MAC layer, the RLC layer and the PDCP layer.
The physical layer of the first layer (L1) and the MAC and RLC layers of the second layer (L2) are the same as those illustrated in FIG. 2. A PDCP layer of the second layer (L2) performs header compression to reduce the size of a relatively-large IP packet header containing unnecessary control information in order to effectively transmit IP packets, such as IPv4 or IPv6, within a radio-communication period having a narrow bandwidth.
Uplink and downlink channels for transmitting data between the network and the UE will hereinafter be described in detail. Downlink channels transmit data from the network to the UE. Uplink channels transmit data from the UE to the network.
Examples of downlink channels are a Broadcast Channel (BCH) for transmitting system information and a downlink Shared Channel (SCH) and a Shared Control Channel (SCCH) for transmitting user traffic or control messages. The user traffic or control messages of a downlink multicast service or broadcast service may be transmitted over the downlink shared channel (SCH) or may be transmitted over an additional multicast channel (MCH).
Examples of uplink channels are a Random Access Channel (RACH) and an uplink shared channel (SCH) and a shared control channel (SCCH) for transmitting user traffic or control messages.
An orthogonal frequency division multiplexing (OFDM) scheme for use in the physical layer which is the first layer will hereinafter be described in detail. The OFDM scheme divides a high-speed data stream into several low-speed data streams and simultaneously transmits the low-speed data streams over a plurality of carriers. Here, each of the plurality of carriers is a subcarrier.
The OFDM scheme has orthogonality between several subcarriers. Therefore, the frequency components of the subcarriers can be detected at the receiving side although frequency components of the subcarriers overlap each other.
The high-rate data stream is converted into several low-speed data streams by a serial/parallel converter, which is also called a “serial to parallel converter.” Each of the subcarriers is multiplied by the parallel data streams generated by the serial/parallel converter and the resultant data streams are summed such that the sum of the data streams is transmitted to the receiving side.
The parallel data streams generated by the serial/parallel converter may be transmitted to a destination over a plurality of subcarriers according to an Inverse Discrete Fourier Transform (IDFT) scheme. The IDFT may be effectively implemented by an Inverse Fast Fourier Transform (IFFT) scheme.
Temporal signal dispersion generated by the multi-path delay spreading, such as relative signal dispersion in time, decreases as symbol duration of the low-speed subcarrier increases. A guard interval longer than a channel delay dispersion interval is inserted between OFDM symbols such that inter-symbol interference may be reduced. If the OFDM symbols can be cyclically extended, a duplicate of some parts of the OFDM signal is arranged at the guard interval, thereby resulting in the protection of the symbols.
A conventional Orthogonal Frequency Division Multiple Access (OFDMA) scheme will hereinafter be described in detail. The OFDMA scheme transmits some pails of subcarriers available for a system based on OFDM modulation to individual users such that a multiple access function may be implemented for the users.
The OFDMA scheme transmits frequency resources called subcarriers to the users. The frequency resources are transmitted to the users independent of each other such that they do not overlap each other. As a result, the frequency resources are exclusively allocated to the users such that there is no overlapping.
FIG. 4 is a block diagram illustrating a transmitter based on a Discrete Fourier Transform Single-Orthogonal Frequency Division Multiple (DFT-S-OFDM) scheme, hereinafter referred to as a “DFT-S-OFDM transmitter.” A conventional DFT-S-OFDM scheme will be described with reference to FIG. 4.
For the convenience of description, a plurality of variables will be defined. Variable “N” is indicative of the number of subcarriers over which OFDM signals are transmitted. Variable “Nb” is indicative of the number of subcarriers for a specific user. Variable “F” is indicative of a Discrete Fourier Transform (DFT) matrix. Variable “s” is indicative of a data-symbol vector. Variable “x” is indicative of a vector generated by data dispersion within a frequency domain. Variable “y” is indicative of an OFDM-symbol vector.
A Single Carrie-Frequency Division Multiple Access (SC-FDMA) system converts the data symbol(s) into a parallel signal using the serial/parallel converter 410 and performs dispersion of the data symbol(s) using a DFT matrix prior to the transmission of the data symbol(s) by the DFT dispersion module 420 the data symbols. Dispersion of is represented by Equation 1:x=FNb×Nbs  [Equation 1]
With reference to Equation 1, FNb×Nb indicates an Nb-sized DFT matrix used for dispersion of the data symbol(s). The subcarrier mapping unit 430 performs subcarrier mapping of the dispersed vector (x) using a specific subcarrier allocation technique.
The IFDT module 440 receives the mapped signal from the subcarrier mapping unit 430 and converts the received signal into a time-domain signal to generate a target signal that is received at the receiving side via the parallel/serial converter 450. The target signal transmitted to the receiving side can be represented by Equation 2:y=FN×N−1x  [Equation 2]
With reference to Equation 2, FN×N−1 indicates an N-sized IDFT matrix required for converting a frequency-domain signal to a time-domain signal. The cyclic prefix insertion unit 460 inserts a cyclic prefix into the OFDM-symbol vector (y) such that the resultant “y” signal including the cyclic prefix is transmitted to a destination.
A method for generating a transmission signal and transmitting the signal to the receiving side according to the above-mentioned scheme is referred to as an SC-FDMA method. The size of the DFT matrix can be controlled in various ways to implement a specific object.
FIG. 5 is a conceptual diagram illustrating a hybrid ARQ (HARQ) scheme. A method for implementing HARQ in the downlink physical layer of a radio packet communication system will be described with reference to FIG. 5.
Referring to FIG. 5, the eNode-B determines a UE that is to receive packets and information of the type of packet that is to be transmitted to the UE, such as a code rate, a modulation scheme and an amount of data. The eNode-B informs the UE of the determined information over an HS-SCCH (High-Speed Downlink Shared Control Channel) and transmits a corresponding data packet (HS-DSCH) at a time associated with the transmission of the information over the HS-SCCH.
The UE receives information of the packet over the HS-SCCH. The UE then recognizes the type and transmission point of the packet and receives the corresponding packet.
The UE transmits a negative acknowledgement (NACK) signal to the eNode-B if the UE fails to decode a specific packet, such as data1. The eNode-B recognizes that packet transmission has failed and re-transmits the same data, such as data1, using the same packet format or a new packet format at a suitable time point. The UE combines the re-transmitted packet, such as data1, and a previously-received packet for which packet decoding failed and re-attempts packet decoding.
The UE transmits an acknowledgement (ACK) signal to the eNode-B if the packet is received and decoded successfully. The eNode-B recognizes successful packet transmission and performs transmission of the next packet, such as data2.
Some parts of the radio resources are allocated according to scheduling of the upper layer in order to allow a plurality of eNode-Bs to communicate with a plurality of UEs over limited radio resources using a mobile communication system. The scheduling is generally executed by the eNode-B(s), which communicate with the UEs using the allocated radio resources.
The eNode-B allocates radio resources to the UE upon receiving a request from the UE. The eNode-B may also allocate the radio resources to the UE if there is a need to allocate radio resources to the UE in the absence of a request from the UE. The radio resources are also allocated to the UE if there is a need to pre-allocate the radio resources to the UE.
The UEs transmit data or control signals using prescribed radio resources allocated by the eNode-B. The radio resources allocated by the eNode-B in an uplink transmission are prescribed for specific UEs.
Therefore, the radio resources may be unnecessarily wasted if the specific UEs do not use the radio resources allocated by the eNode-B. In other words, it is difficult to re-allocate radio resources that were pre-allocated to specific UEs to other UEs, thereby resulting in ineffective use of radio resources.